This invention relates to continuous-feed filtering- or screening-type centrifuges. This invention also relates to an associated centrifuge method.
Industrial centrifugation processes for separating particulate material from various impurities include sedimentation and filtration. Generally, the particulate material is produced as a cake having different degrees of moisture depending on the type of particulate material and the particular separation process. The cake constitutes a heavy phase whereas the impurities are removing in a filtrate constituting a light phase.
A decanter-type centrifuge has a conveyor in the form of one or more helical screw wraps rotating at a slightly different angular velocity from the velocity of the bowl or outer wall. Where the bowl has a solid wall with a cylindrical shell followed by a conical shell and extends from a clarifier pool at an input or feed end of the centrifuge to a cake discharge opening or openings at an output end of the centrifuge, the centrifuge is known as a decanter or a solid bowl. A sedimentation process occurs in the cylindrical portion of the centrifuge and a dewatering of the cake in the conical dry beach area. Where the bowl is provided with one or more screen sections downstream and outside of the clarifier pool, the decanter-type centrifuge is known as a screenbowl centrifuge and performs a filtration process.
Another kind of filtration centrifuge is a pusher or pusher basket. Such a centrifuge includes a first cylindrical basket at an input end of the centrifuge and a second cylindrical basket of greater diameter at a cake output end of the centrifuge. The baskets rotate at a high angular speed. In addition, the baskets of this two-stage basket system are longitudinally reciprocatable relative to one another, whereby pusher plates shove the heavy phase particulate material in a layer along the first basket, from the first basket to the second basket, and along the second basket to a cake discharge port. Single-stage pushers or pushers with two-or-more stages such as quadruple-stage pushers are also available.
Filtering centrifuges have been used to wash the cake to remove the impurities. There are two types of washing: a spray wash and a flood wash. In a spray wash, wash liquid is applied to a localized area on the cake surface in an attempt to displace mother liquor which contains the impurities. Spray washing is used most commonly in a screenbowl centrifuge where the cake height varies across the screen from a thin layer to a thick layer adjacent to the pressure face of the conveyor blade.
Another kind of centrifuge, used particularly for the dewatering and washing of thickened slurries with particulate solids, is a conical-screen centrifuge. The centrifuge wall includes a conical screen which has an increasing diameter in the cake flow direction. The particulate solids are held by the screen as the liquid filters through. The conical screen has the advantage that the cake experiences an increasing centrifugal gravitational force as the cake travels down to the large diameter of the cone. The centrifugal gravity is proportional to the radius of the screen for a given rotational speed of the basket. Another advantage of the increasing-diameter conical screen is that, for a given cake mass, the cake height is reduced as the cake moves towards the large-diameter end of the cone, owing to the conservation of mass. Both of these advantages enhance the dewatering of the cake. Also, spray washing is used in conical-screen centrifuges to remove impurities dissolved in the mother liquor.
In a conical-screen centrifuge, a thickened or concentrated feed is introduced, after pre-acceleration to the proper tangential speed, into the centrifuge at the smaller end of the conical screen. The cake travels down the cone when the half cone angle, typically 30.degree. to 40.degree. with respect to the axis of the machine, is steep enough to overcome frictional forces.
When the cone angle is small, typically 15.degree. to 25.degree., a mechanical conveyance mechanism is used to convey, the cake from the small end of the cone to the large end thereof One mechanism is a helical screw conveyor with a single continuous lead. Another, related, mechanism is a multiple-lead screw conveyor (4 leads is common). Yet another mechanism is a set of discrete scraper blades each conforming to a helix. In any case, the conveyor rotates at a differential speed as compared to the screen, thereby conveying cake down the screen. By adjusting the differential speed, the cake movement and concomitantly the cake residence time can be adjusted. Another mechanism is a vibrator, such as rotation of eccentric weights with an axis of vibration parallel to the axis of the machine. The inertia force generated by the vibration propels the cake from the small end to the large end, the discharge end, of the centrifuge.
FIG. 1 is a schematic drawing of a conventional screenbowl centrifuge. A bowl 20 has a solid cylindrical portion 22 at an input end of the centrifluge. Input of a slurry is effectuated via a feed pipe 24 and an outlet opening 26 into a feed compartment 24a where the feed is accelerated to tangential speed before it is discharged through feed ports 26a. The accelerated slurry is fed to an annular pool 28 in which clarification and sedimentation occurs. Pool 28 is located in the solid cylindrical portion 22 of bowl 20 and overlaps a solid beach portion 30 of the bowl. After a cake layer 32 is formed, it is conveyed by a helical conveyor blade 34 out of the pool 28 up the conical beach 30 to a cylindrical screen section 36. In a first portion of the screen 36, wash liquid 38 is introduced to remove the mother liquor containing the dissolved impurities.
FIG. 2 is a schematic cross-section of the helical conveyor blade 34 and screen 36 and shows a cake profile 40 formed adjacent to a pressure face 42 of the blade. Wash liquid 38 is applied through a wash nozzle assembly 44 (FIG. 1) or 44' through a conveyor hub 46. A remaining portion of the screen 36 is used for ultimate dewatering of both the mother liquor and more importantly the wash liquid which displaces the mother liquor in the first portion of the screen so as to obtain both pure and dry cake from the centrifuge.
Improved spray wash nozzle designs and methods disclosed respectively in U.S. Pat. No. 5,403,486 and U.S. Pat. No. 5,527,474 provide wash liquid which has a wider area of coverage on the cake surface and has a tangential velocity matching that of the cake so that the wash liquid attains the same centrifugal gravity as that of the cake. Otherwise, the wash liquid enters the centrifuge as a lighter fluid situated above the heavier cake profile and does not penetrate and wash the cake.
On the other hand, flood washing is more effective in providing a pool of wash liquid used to displace the mother liquor. Flood washing is most commonly used when operating with a uniform cake such as found in a continuous pusher or batch baskets. However, flood washing still does not provide a satisfactory wash at times insofar as some of the areas in the porous cake can be excluded from being swept such as dead pockets or zones. Also, fingering or channeling further reduces the wash liquid sweeping efficiency. In contrast, spray washing does not warrant wash on a larger surface area as with a flood wash. In addition, given the angle of repose (see FIG. 2), the wash liquid runs down the cake to the screen section, bypassing the cake interior entirely. This result is more probably the case when the wash liquid is not accelerated to speed. Even when the wash liquid is accelerated to the speed of the cake, a jet of wash liquid with high radial momentum is likely to shoot the cake solids, especially the fines, through the screen, thereby defeating the purpose of the spray wash.
For porous cake with a rough particle surface, the impurities are trapped and it becomes difficult to remove these impurities with either conventional spray washing or flood washing. As illustrated in FIG. 3, the cake is conventionally reslurried in a mixing or repulping tank 48 and sent to a centrifuge 50 for dewatering. A feed concentrate with high impurities is delivered to repulping tank 48 via an inlet 52, while a wash liquid with low impurities is input at 54. The reslurried cake is conveyed from repulping tank 48 to centrifuge 50. Centrifuge 50 outputs purified cake with low impurities and a filtrate or spent wash liquid, respectively represented by arrows 58 and 56. As shown in FIG. 4, this reslurrying-and-separation may repeated in one or more subsequent stages, each stage including a mixing or repulping tank 60 and a centrifuge 62, until the impurities are removed to within specification. Filtrate from a subsequent centrifuging operation may be fed back to a repulping tank, as indicated at 64.